Leetcode: 202. Happy Number

Write an algorithm to determine if a number n is happy.

A happy number is a number defined by the following process:

• Starting with any positive integer, replace the number by the sum of the squares of its digits.
• Repeat the process until the number equals 1 (where it will stay), or it loops endlessly in a cycle which does not include 1.
• Those numbers for which this process ends in 1 are happy.

Return true if n is a happy number, and false if not.

Example 1:Input: n = 19 Output: true Explanation: 1² + 9² = 82 8² + 2² = 68 6² + 8² = 100 1² + 0² + 0² = 1

Example 2:Input: n = 2 Output: false

Constraints:

• 1 <= n <= 231 - 1

By using set, we can check whether or not the number has apeared before. If not, then we just keep calculating until it repeats or get 1. Otherwise, return false.

class Solution:
    def isHappy(self, n: int) -> bool:
        if n <= 0:
            return False
        
        check_set = set()

        while True:
            if n in check_set:
                return False
            
            check_set.add(n)

            n = sum([int(i) ** 2 for i in str(n)])

            if n == 1:
                return True